Consumers are increasingly demanding both faster and smaller electronic products. The use of PCBs has grown enormously as new electronic applications are marketed. A PCB is formed by laminating a plurality of conducting layers with one or more non-conducting layers. As the size of a PCB shrinks, the relative complexity of its electrical interconnections grows.
A via structure is traditionally used to allow signals to travel between layers of a PCB. The plated via structure is a plated hole within the PCB that acts as a medium for the transmission of an electrical signal. For example, an electrical signal may travel through a trace on one layer of the PCB, through the plated via structure's conductive material, and then into a second trace on a different layer of the PCB.
Unfortunately, due to limitations within the prior art, the plated via structure may be longer than necessary to perform the function of electrical connectivity. For example, the plated via structure may extend completely through the PCB but only connect two traces on two proximate adjacent layers. As a result, one or more stubs may be formed. A stub is excessive conductive material within the plated via structure which is not necessary to transport the electrical signal.
When a high speed signal is transmitted through the plated via structure, a “stub effect” may distort the signal. The stub effect is a result of the useless excess conductive material present within the plated via structure. The stub effect occurs when a portion of the signal is diverted away from the trace connections and into one or more stubs of the plated via structure. The portion of the signal may be reflected from the end of the stub back toward the trace connections after some delay. This delayed reflection may interfere with signal integrity and increase, for example, the bit error rate of the signal. The degenerating effect of the stub effect may increase with the length of the stub.
FIGS. 1A-1E illustrate the typical stages of forming back-drilled hole in a printed circuit board (PCB). As shown, the PCB 100 includes stacked material insulator layers (typically laminates and prepregs) 104a, 104b . . . 104f separating five internal layers 105a, 105b . . . 105e and two external layers 108a and 108b. The internal layers 105a, 105b and 105e are signal layers and the internal layers 105c and 105d are plane layers. The PCB 100 has an upper surface 110a and an opposing lower surface 110b. 
To form the plated through hole 101 as illustrated in FIG. 1E, a first hole 102 having a first diameter d1 is drilled through the PCB 100 (see FIG. 1A). Next, a second hole 103 having a second diameter d2 is drilled concentrically around and through a predetermined depth of the first hole 102 (see FIG. 1B). Then, the walls of the remaining portion of the first hole 102 and the walls of the second hole 103 are plated with a thin layer of a conductive material 106 (e.g., copper). (See FIG. 1C).
Next, a drill 112 having a diameter d3 is used to back-drill and form a third hole 104 concentrically around and through the remaining portion of the plated first hole 402 starting from the lower surface 110b of the printed circuit board 100 (see FIG. 1D). Then the drill 112 is removed from the printed circuit board 100 which now has the via 101 formed therein that includes a first through hole 107 (e.g., plated second hole 103) and a back-drilled hole 109 (e.g., third hole 104) (see FIG. 1E).
However, when back-drilling the drill bit has no pressure and it pushes up against the traces on the top and as a result fluctuating is created as well as peel off of the conductive plating occurs. (See FIGS. 1D-1E) Consequently, there is a need for improved methods for removing stubs when forming plated through holes in a PCB.